fish, the friendly interactive shell, is a commandline shell intended to be interactive and user-friendly.
fish is intentionally not fully POSIX compliant, it aims at addressing POSIX inconsistencies (as perceived by the creators) with a simplified or a different syntax. This means that even simple POSIX compliant scripts may require some significant adaptation or even full rewriting to run with fish.
source, I use debian with fish and I write my scripts still in POSIX since fish is god slow at scripting, it is really nice for interactive usage but scripting performace is bad, and I can’t assume bash to be everywhere so POSIX I go